Excuse me, what does the hollow mark on the floor plan of high-rise buildings mean?

literally means that this place is empty and there are no buildings or structures. For example, a hotel, entering the lobby, the top of the lobby is very high, with a height of three floors. Then, on the drawing of this hotel, on the second floor plan, the location of the lobby is actually empty, so this place should be marked as hollow. In addition, pipes such as garbage passages and ventilation ducts are sometimes marked hollow on the plan.